Optimized Control in Grid Connected Photovoltaic System based on Single-Stage Voltage Source Inverter (original) (raw)

Among the renewable energy sources, a noticeable growth of small PV power plants connected to low-voltage distribution networks is expected in the future. A single-stage inverter, controlled by using the one-cycle control (OCC) technique, has been presented in the literature. The theoretical analysis of such an inverter, reported in, is not in-depth. Thus, it does not allow satisfactory performance both in terms of power extracted from the PV array and quality of the current injected into the grid., wherein a significant improvement of the theoretical model has been proposed and used to increase the power extracted from the PV model and to reduce the distortion of the current injected into the single phase grid. Simulation results have demonstrated that the set of variables influencing the operation of the one-cycle controller can be optimized to get nearly maximum power at one assigned insolation level or the highest average power in a given insolation range.. Moreover, it has also been highlighted that the inverter proposed in [1] is lacking a true maximum power point tracking (MPPT) controller. After values of the parameters of the analog circuitry implementing the OCC are fixed, power extracted from the PV array is maximized at only one specific irradiation level. Therefore, under time varying atmospheric conditions, a consistent decrease in efficiency of PV power extraction is observed. To overcome such a limitation, [6] proposed the idea of matching a digital MPPT controller with analog OCC circuitry, and some preliminary simulation results have been presented. Environmentally benign technologies like solar photovoltaic (PV)-based systems are increasingly being used for electricity production in the context of global warming, climate change, and rapid exhaustion of fossil fuels. A portion of the huge gap between the expected demand and availability of the electricity produced in many parts of the world, particularly in developing countries, is expected to be met from renewable energy sources like solar PV. (Online) 137 | P a g e of the inverter output voltage is derived by processing the inverter switching function and the dc-link voltage through an analog filter and a saturator. The idea of estimating the grid voltage or virtual flux, utilizing the switching function of the inverter, grid Marimuthu et al. (Online) 139 | P a g e instability in OCC-based inverter. It synthesizes the fictitious current signal required by multiplying the fundamental component of the inverter output voltage with a constant gain. Information regarding the inverter output voltage is obtained from the switching function used to trigger the inverter switches and not by sensing the inverter output voltage. The schematic control block diagram of the proposed scheme is shown in . The dc-link capacitor voltage is sensed and compared with a set reference, and the error so generated is fed to a proportional and integral regulator to produce a signal Vm. A saw tooth waveform of constant frequency having a peak-to-peak value of 2Vm is generated using a resettable integrator. A free-running clock having a time period Tsis used to reset the integrator, and hence, the frequency of the clock Fs decides the frequency of the saw tooth waveform as well asthe switching frequency of the devices. The time constant of the integrator Ti is chosen to be half of Tsas explained. A fictitious current signal proportional to the fundamental component of the output voltage of the inverter (if = VI1/Rp) is added with the source current and properly scaled to obtain the modulating signal x, where x = i s +i f = i s In order to obtain VI1 and hence if , inverter switching pulses are passed through a saturator. The output of the saturator pulsates between the scaled dc-link voltage (Vdc) and zero in tandem with the pulsation of the switching sequence between the states one and zero. The signal proportional to VI1 is obtained by filtering the output of the saturator. The harmonic spectrum of the saturator output has: 1) a fundamental frequency component (50 Hz); 2) a dc component; and 3) higher frequency components centeredaround multiples of switching frequency. Hence, a band pass filter (BPF) is required to retrieve the fundamental component of this signal and filter out the dcand higher order components. A second-order BPF having a central frequency equal to the power frequency (50 Hz) is used for the purpose. The circuit diagram of the second-order filter is shown in Fig. 4. The modulating signal is multiplied by a gain Rsand is then compared with the saw tooth waveform to generate the switching pulses. At every rising edge of the clock pulse, S3 and S4 are turned on which leads to the increment in source current is. When the modulating signal becomes equal to the saw tooth waveform, S3 and S4 are turned off and S1 and S2 are turned on so that the modulating signal and hence is decrease. The rising and falling slopes of are given by $ (vs+ Vdc)/L and (vs − Vdc)/L, respectively, where vsis utility voltage, Vdc is the dc-link capacitor voltage, and L is themagnitude of the boost inductor.